This sentence appears in Donna Leon's Acqua Alta, Chapter 20:
Auf dem zweiten Treppenabsatz hörten sie die Musik durchs Treppenhaus heruntertönen.
The word "heruntertönen" does not appear in any online resource, Duden, Pons, Reverso, Leo, dict.cc, nor in DWDS. Google translate tells me, "tone down," but that makes little sense to me in the context. So my question is, What does a native German speaker do in this situation, with a word he may have never seen before because it is not part of any standard lexicon?
I imagine that such a German speaker would look at the parts and infer that the author is trying to convey the idea that the sound of the music is coming down the stairwell from above. Is that an accurate understanding and is it a complete one, or is there more to it than that?
If that is the case then it would seem that German authors may be expected to make up words ad lib, that have no particular meaning by themselves but, instead, must always be interpreted in the context in which they appear, as inferred from parts that do have particular meaning on their own. Is this true?